Radar systems are widely used for detecting and locating objects. Radar systems usually operate in the UHF (Ultra-High-Frequency) or microwave part of the RF (Radio-Frequency) spectrum, and are used to determine the position and/or movement of an object. There have been developed various types of radar systems, for different purposes and applications.
Any radar system can locate a target by finding its direction and range. The range to an object is determined by calculating the delay between a pulse transmitted by the system, and the consequential receipt of the reflection of said pulse from the object. This determination is based on the known propagation velocity of the pulse c (the speed of light), when the air is the medium. As a general rule, for obtaining a relatively good range resolution, a very short pulse should be transmitted. The duration of the pulse may be shortened by any known compression technique.
Metal junctions and electronic components, particularly those containing semiconductors, have a non-linear response to the application of voltage over them. This physical phenomenon is known in the art for several decades, and has been used, for example, in systems for detecting and locating mines. In these cases, the current I via a metal or a semiconductor junction can be expressed by the following equation:I=I0+g1V+g2V2+g3V3+ . . .   (1)
The coefficient g1 in this equation expresses conductance, while the other terms represent the coefficients of the non-linear response of the object to the application of voltage over it. Typically, the values of the coefficients gn decrease as the order of the coefficient index n increases. The said phenomenon is used by object detection systems to remotely detect and characterize man-made objects which are made of metals or contain semiconductor components. Generally, such systems transmit electromagnetic waves towards the object, that induce voltages over the metal or the semiconductor junctions of it. These voltages induce currents according to equation (1), which cause radiation that can be detected by a receiving portion of the system. The radiation contains information regarding the non-linear characteristics of the object, that are characterized by the distinct relative levels of coefficients gn.
In most radar applications, a sinusoidal or a quasi-sinusoidal wave is transmitted towards the object. Assuming that a wave V cos(2πft) is transmitted, the scattered non-linear response from a man-made object contains harmonics of the transmitted frequency f, namely, 2f, 3f, 4f, 5f, . . . . The reflections from natural objects, however, are linear, i.e., include only the fundamental transmitted frequency f. Also, if the transmission contains more than one frequency, the response contains various combinations of the transmitted frequencies.
Man-made objects that do create harmonic scattering can be divided into two classes:
One. Man-made metal objects: Due to an oxidized layer formed on most metals, junctions of metals are essentially combinations of metal-oxide-metal (MOM), which cause a symmetrical non-linear re-radiation of the applied voltage (by “symmetrical”, it is meant that I(V)=−I(−V)). Due to the symmetrical structure of man-made metal objects, the harmonic response contains only odd harmonics: 3f, 5f, 7f, . . . .Two. Man-made semiconductor junctions: Semiconductor junctions, existing, for example, in diodes and transistors, are essential to the operation of electronic instruments, and cause non-linear re-radiation. As is known, this non-linearity is not symmetrical, namely, |I(V)|≠|I(−V)|. Therefore, the scattering from electronic components contain the fundamental frequency and its entire harmonics, odd and even, i.e., 2f, 3f, 4f, 5f, 6f, 7f . . . .
From the above, it is clear that a harmonic radar can detect man-made objects, and can even distinguish between metal objects and objects that contain electronic components.
A few general properties of the harmonic response should be noted:
One. Frequency dependence: Physically, there is no known low boundary to the linear response from typical objects. In practice, it is difficult to concentrate low frequency radiation on targets, and also the coupling is not efficient (the term “coupling” refers to the ratio between the voltage induced on a component inside the target object and between the intensity of the electromagnetic field surrounding the target object). On the other hand, parasitic capacitance of the junctions that are part of the object, for example, PN junctions in semiconductor components within the object, metal-semiconductor junctions, or metal-oxide-metal junctions, tend to short the voltage in high frequencies. More particularly, as the frequency increases, a lower amplitude response is scattered from a man-made object. Harmonic radar systems using frequencies ranged from a few tens of MHz up to about 10 GHz have been applied for various radar applications. However, second harmonic generation by metal-oxide-metal (MOM) diode made by dissimilar metals or metal-oxide semiconductor (MOS) diodes have been demonstrated up to 30 THz (teraherz) using CO2 laser (by F. Kneubel et al., ETH, Zurich).Two. Power Dependence: As a general rule, the re-radiation from man-made objects in each harmonic is proportional to the incident power raised by the n-th power, n is approximately equal to the harmonic order. For example, if the incident power is P, the scattering from the object in the second harmonic 2f is approximately proportional to P2.Three. Order Dependence: The higher the harmonic order, the lower the response from the object.
Harmonic radar systems utilizing the above phenomena are used in the art, generally for short-range detection and location of objects, typically in ranges of between several centimeters, up to several kilometers. Such systems, when applied to detect objects in ranges of several kilometers require transmission of very high power.
Parasitic harmonic signals that are developed in the transmitting unit cause significant problems in distinguishing the searched objects from their surrounding. For example, the operation of a transmitter is accompanied by many non-linear electromagnetic components, particularly in the amplifier or the oscillator, that produces radiation. Also, metal junctions in the antenna and the transmission lines may create odd harmonics. Furthermore, corona formation in the transmitter caused by the high transmitted power might create harmonics of all orders. Harmonics of the fundamental transmitting frequency which are produced in the transmitter are then transmitted towards the object, together with the main transmission in the fundamental frequency. These transmitted harmonic frequencies are scattered by all objects and may mask the expected scattered harmonic signals from the non-linear objects, and prevent their detection and investigation. Moreover, the transmitter platform may contain metallic materials and electronic devices which may, by themselves generate harmonic signals, that may be confused with the harmonics scattered by the objects. This is a particular problem when using antennas having strong side-lobes, that cause problems in short-range detection in any type of radar system.
WO 02/39140 (Jablonsky et. al.) discloses a harmonic radar nonlinear junction detector system for detecting concealed weapons, electronics, and other man made objects. Jablonsky suggests using transmission pulses having durations between about 0.3 msec (B=3000 Hz), to about 30 msec (B=30 Hz), or up to about 0.1 seconds.
U.S. Pat. No. 6,163,259 (Barsumian et. al.) discloses a non-linear junction detector, which transmits a pulse having a width of 1.5 msec.
It is an object of the invention to provide a method and system for detecting and locating man-made objects using harmonic detection.
It is another object of the invention to provide a harmonic radar method and system for detecting and locating man-made objects at close-range.
It is still an object of the invention to provide a harmonic radar method and system enabling better distinction of the searched man-made objects from their surroundings, particularly when parasitic harmonics are generated at the transmitting and receiving units, and at the carrying platform.
It is a particular object of the invention to provide a harmonic radar method and system for locating man-made objects in very high precision, namely, with high range and azimuth resolution.
It is still a particular object of the invention to provide a method and system for precisely locating man-made objects buried in the ground.
It is still a particular object of the invention to provide a method and system for remotely classifying man-made objects. The term “classifying” refers herein to the ability to distinguish and classify the type of the target detected, for example, to distinguish between a mine, tank, missile, etc.
Other objects of the invention will become apparent when the description proceeds.